The ancient Greeks eventually settled on an alphabetic number system, where the first nine letters were used to represent numbers 1-9 (they had no concept of zero), the next nine represented multiples of ten (10, 20, 30,...) and the next nine represented multiples of one hundred (100, 200, 300,...). To represent thousands, the ancient Greeks used the first nine with a special tick mark at the bottom left.
So for example, let's say you want to write the number one hundred twenty three (123). The symbol used for 100 was ρ, the symbol for twenty was κ and the symbol for 3 was γ, so the notation would become: ρκγ
The Greeks had no decimal symbol and no knowledge of place value, but they did know of fractions and had a notation for them: (numerator)'(denominator)''(denominator)''. So for example, if you want to represent the fraction four fifths note that the numerator is 4, and the symbol for that is δ, and the denominator is 5, for which the symbol is ε, so you would write the fraction like this: δ'ε''ε''
In order for this calculator to represent decimals, it first has to convert the decimal into a simplified fraction and then convert the integer numerator and denominators into Greek notation, and then assemble it into the fraction notation shown above.
If you want to learn more about this number system, an excellent resource is the MacTutor entry on Greek number systems.